Is the expert system still in use today?
I would say Expert Systems is still being taught. For instance, if you look at some of the open courses like MIT's, there are still lectures on it.

Also, looking at the CLIPS documentation, you will find a couple of examples of usage from 2005. 

What I suspect is that Expert Systems are now embedded with "normal systems" in practice. Hence it may be difficult to distinguish from systems used on a daily basis for diagnostics, etc. and not as popular as before.